In geotechnical engineering, it is often necessary to determine the denseness of soil, generally sandy soils, in order to determine permissible loads, possible settlements, earthquake liquifaction potential, densification control, etc. One of the methods used to determine soil density is dynamic cone penetration testing. In this test a fixed weight is dropped for a predetermined distance onto a striking surface of a cone rod which has a penetrating cone at the bottom tip inserted into the soil. The number of blows required to advance the penetrating cone for a predetermined depth into the soil becomes an index of the denseness of the soil. The number of blows generally runs between about 5 and 75 blows per foot of depth. For different soils blow counts can vary from 1 to as high as about 200 for one foot of depth.
In the past friction along the cone rod is reduced by having a cone at the penetrating tip with a larger diameter than the cone rod. This reduces the friction and the method has been proved successful because it is fast, simple and reasonably reliable at shallow depths.
As the depth of soil becomes greater and one is determining density at greater depths, then friction starts to build up between the soil and the rod so that the blows from the drop hammer on the striking surface of the rod no longer reflect the same force on the penetrating tip and thus the number of blows per foot does not have the same meaning because friction has introduced a restriction.
Webster in U.S. Pat. No. 5,313,825 illustrates a penetrometer of the type known in the prior art having a hammer and a penetrating tip at the bottom end of a rod. Diggle in U.S. Pat. No. 4,499,954 shows a method of lubricating a string of hollow rods for a cone penetrometer to attempt to overcome friction between the rod and the soil within the probe hole.
It is an aim of the present invention to reduce the rod friction as the penetrating cone is being driven into the soil so that the number of blows per foot of depth becomes a substantially accurate index whether the test occurs in a shallow or deep probe hole.
This aim of the present invention is achieved by rotating or spinning the rod during the driving advancement. This spinning action is achieved by means of a fixed position motor rotating the cone rod as it is being driven into the soil.